Saponified (solution type) rosin sizing agents have been used for internal addition in a process for manufacturing paper under acidic conditions together with the use of aluminum sulfate. However, such a sizing agent exerts a poor sizing effect at a low addition level. Moreover, the sizing effect is further deteriorated due to an increase in water temperature accompanying the recent closed drainage system or within a range around neutral pH. In order to overcome the above-mentioned disadvantages of the saponified rosin sizing agents, emulsion type rosin sizing agents (JP-A-60-133052, JP-A-2-33393 and JP-B-2-53555, the term "JP-A" as used herein means an "unexamined published Japanese patent application", while the term "JP-B" as used herein means an "examined Japanese patent publication") and sizing agents obtained by the alkali-saponification of an alkenylsuccinic anhydride or an alkenylsuccinic acid (JP-A-58-214598) have been proposed. However, these sizing agents suffer from the problem that the sizing effects thereof are deteriorated in papermaking at a high temperature or within a range around neutral pH, similar to the saponified rosin sizing agents.
Regarding alkenylsuccinic anhydride emulsion sizing agents used mainly for neutral papermaking (U.S. Pat No. 3,821,069, JP-B-53-36044, JP-A-57-154495, JP-A-60-99098 and JP-B-4-53995), a method wherein an alkenylsuccinic anhydride containing an emulsifier is emulsified with a solution of cationic starch or water is known. In addition, methods wherein an alkenylsuccinic anhydride is emulsified by using an aqueous solution of an acrylamide-series polymer have recently been proposed (JP-A-2-47393 and JP-A-6-116894).
The reaction responsible for alkenylsuccinic anhydride sizing is as follows. Sizing is achieved by the anhydride group in the alkenylsuccinic anhydride molecule esterifying a fiber hydroxyl group of cellulose, i.e., the main component of pulp, and thus, introducing the substituent hydrophobic alkenyl group onto the fiber surface. Because of the higher reactivity of alkenylsuccinic anhydride, it is known to form an ester bond with the hydroxyl group of cellulose. In the conventional neutral papermaking process, it is therefore necessary that the alkenylsuccinic anhydride be added to a pulp slurry, in the form of the anhydride. However, the alkenylsuccinic anhydride is highly reactive with water. When the alkenylsuccinic anhydride is preliminarily emulsified and dispersed in water, therefore, it reacts with water within a short period of time. As a result, an acid anhydride group is converted into a carboxyl group which causes a loss in the function as a neutral sizing agent. During the process of the conversion of the acid anhydride group into the carboxyl group due to the reaction with water, the emulsion system is changed with aggregation, precipitation and separation occurring. Therefore, an emulsion sizing agent comprising an alkenylsuccinic anhydride for neutral papermaking in the form of an aqueous dispersion can be stored for only several hours. Accordingly, it cannot be supplied in the form of an emulsion product with a high concentration. This characteristic requires that it be emulsified and dispersed by using an emulsifier immediately before the papermaking.
It is also known that an alkenylsuccinic anhydride shows a lower esterification reactivity of the acid anhydride group under acidic conditions than under neutral or weakly alkaline conditions. When an emulsion of an alkenylsuccinic anhydride is used under acidic papermaking conditions (in particular, at a pH value of 5.0 or below) the sizing effect appears slowly, and only a poor sizing effect can be achieved immediately after the papermaking. If an alkenylsuccinic anhydride could be hydrolyzed into the corresponding alkenylsuccinic acid and then emulsified, the alkenylsuccinic acid would rapidly react with aluminum sulfate to thereby achieve a good sizing effect. Thus the above-mentioned problem in the reactivity of the conventional alkenylsuccinic anhydrides under acidic conditions might be solved. However an alkenylsuccinic acid can be hardly emulsified due to its highly hydrophilic nature. It is therefore difficult to obtain an emulsion which has a higher concentration than the conventional rosin emulsion sizing agent concentrations and which can remain stable over a long period.
As the results of recent studies, an emulsion sizing agent has been proposed wherein an alkenylsuccinic acid obtained by reacting a branched internal olefin with maleic anhydride is dispersed in water by using an anionic high-molecular weight dispersion stabilizer optionally together with an emulsifier (JP-A-5-140891). However it is very difficult to emulsify such an alkenylsuccinic acid obtained from a branched olefin and, therefore, it is necessary to use a highly emulsifiable substance together with it. On the other hand, an alkenylsuccinic acid, which is obtained from a product of a reaction between a linear internal olefin and maleic anhydride, can be easily emulsified, different from those obtained from branched olefins, but shows only a poor or no sizing effect (JP-A-6-128896).